Quick: Rely on inferred types in GVN/LVN/DCE.
Fix LVN::GetEndingVregValueNumberImpl() to check whether
the requested wideness matches the SSA register type as
recorded in MIRGraph::reg_location_.
Add DCHECKs that the wideness matches when getting/setting
sreg values, update Phi handling in LVN/DCE to use the type
from MIRGraph::reg_location_ instead of determining it from
the sreg value maps which would now trigger the DCHECKs.
Update tests to initialize MIRGraph::reg_location_.
Reenable DCE.
Bug: 20572509
Change-Id: I1a4d4e32cd57807ca8b56d2f3ed5e1288660b82e